Trusted Communications with Physical Layer Security for 5G and Beyond

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Affiliations:
1: School of Electronics, Electrical Engineering and Computer Science, Queen's University Belfast, UK
2: Research School of Engineering, The Australian National University, Australia
3: Department of Electrical Engineering, Princeton University, USA

Part I: Fundamentals of physical layer security

Physical layer security over fading channels has drawn a considerable amount of attention in the literature. How to measure the secrecy performance is a fundamental but important issue for the study of wireless physical layer security. This chapter gives a comprehensive overview of the classical secrecy metrics as well as several new secrecy metrics for physical layer security over fading channels, which enables one to appropriately design secure communication systems with different views on how secrecy is measured.

Recent years have been marked by an enormous growth of wireless communication networks and an extensive use of wireless applications. In return, this phenomenal expansion is inducing more concerns about the privacy and the security of the users. For many years, the security challenge has been mainly addressed at the application layer using cryptographic techniques. However, with the emergence of ad-hoc and decentralised networks and the deployment of 5G and beyond wireless communication systems, the need for less complex securing techniques had become a necessity. It is mainly for this reason that wireless physical layer security has gained much attention from the research community. What distinguishes information-theoretic security compared to other high-layer cryptographic techniques is that it exploits the randomness and the fluctuations of the wireless channel to achieve security at a remarkably reduced computational complexity. However, these technical virtues rely heavily on perhaps idealistic channel state information assumptions. In this chapter, we look at the physical layer security paradigm from the channel uncertainty perspective. In particular, we discuss the ergodic secrecy capacity of wiretap channels when the transmitter is hampered by the imperfect knowledge of the channel state information (CSI).

This book chapter has described a radio resource allocation framework to optimize both the confidentiality and the energy efficiency of a communication system. To this end, a key step has been the introduction of the new performance metric of SEE, defined as the ratio between the system secrecy capacity (or achievable rate) and the consumed power, including both radiated and hardware power.The chapter also features the application of the proposed framework to the special case of a MISOSE system, in which the legitimate receiver and the eavesdropper are equipped with only one antenna.

Part II: Physical layer security for multiple antenna technologies

Antenna selection, which has been recognised as an important technique to enhance physical layer security in multiple-input multiple-output wiretap channels, requires low feedback overhead and low hardware complexity. In this chapter, antenna selection strategies in different application scenarios for wiretap channels are reviewed in order to demonstrate their benefits in terms of enhancing secure transmissions. The basic idea of transmit antenna selection (TAS) is first detailed and then TAS with Alamouti coding is provided. Furthermore, antenna selection strategies in full-duplex wiretap channels are presented, followed by a discussion on the impact of imperfect feedback and correlation on the secrecy performance of TAS.

The chapter is organised as follows. In Sections 5.1 and 5.2, we briefly review the fundamentals of massive MIMO and physical layer security, respectively. In Section 5.3, we motivate the chapter by illustrating why we consider physical layer security for massive MIMO systems. The considered model and the performance evaluation metrics for secure massive MIMO systems are introduced in Sections 5.4 and 5.5, respectively. Section 5.6 studies linear precoding for downlink secure massive MIMO transmission. In Section 5.7, we conclude with abrief summary of this chapter.

This chapter provides some basic concepts of the physical layer security for massive MIMO and jamming attacks. The effect of the jamming attacks on pilot contamination is analysed. Potential counter-attack strategies, which can enhance the robustness of massive MIMO against jamming, are also presented.

In this chapter, we will present criteria for user selection and relay selection to enhance the physical layer security in multiuser multi-relay networks. Specifically, for multiuser networks with multiple amplify-and-forward (AF) relays, we present three criteria to select the best relay and user pair. Criteria I and II study the received signal-to-noise ratio (SNR) at the receivers and perform the selection by maximising the SNR ratio ofthe user to the eavesdroppers. To this end, criterion I relies on both the main and eavesdropper links, while criterion II relies on the main links only. Criterion III is the standard max-min selection criterion, which maximises the minimum of the dual-hop channel gains of main links. For the three selection criteria, we examine the system secrecy performance by deriving the analytical expressions for the secrecy outage probability. We also derive the asymptotic analysis for the secrecy outage probability with high main-to-eavesdropper ratio.

This chapter will introduce trusted communications in multiuser multiple-input multiple-output (MIMO) wireless systems via spatial multiplexing. We will start by considering the multiple-input single-output broadcast channel with confidential messages (BCC) under Rayleigh fading, where a multiantenna base station (BS) simultaneously transmits independent confidential messages to several spatially dispersed malicious users that can eavesdrop on each other. We will then present the broadcast channel with confidential messages and external eavesdroppers (BCCE), where a multiantenna BS simultaneously communicates to multiple malicious users, in the presence of a Poisson point process (PPP) of external eavesdroppers. Unlike the BCC, in the BCCE not just malicious users, but also randomly located external nodes can act as eavesdroppers. We will finally turn our attention to cellular networks where, unlike the case of isolated cells, multiple BSs generate intercell interference, and malicious users of neighboring cells can cooperate to eavesdrop. For these involved scenarios, we will present low-complexity transmission schemes based on linear precoding that can achieve secrecy at the physical layer, discuss their

Part III: Physical layer security with emerging 5G technologies

The main purpose of this chapter is to present the achievable secrecy performance of power beacon (PB)-assisted wirelessly powered communication systems. We first introduce the PB-assisted wiretap channel model and then present an analytical study on the achievable secrecy outage performance assuming a simple maximum ratio transmission (MRT) beamformer at the information source. Later on, the optimal design of the transmit beamformer is investigated. Next, the PB is exploited to act as a friendly jammer to further enhance the secrecy performance of the system. We conclude the chapter with a summary and present potential interesting directions in the area.

Device-to-device (D2D) communication, which enables direct communication between two mobile devices that are in proximity, is regarded as a promising technology for the next generation cellular networks. In this chapter, we focus on the physical layer security issues for D2D-enabled cellular networks. In Section 10.1, we introduce the background of D2D communication, and in Section 10.2, we review the state-of-the-art research on physical layer security for D2D-enabled cellular networks. In Sections 10.3 and 10.4, we study how D2D communication can affect the secrecy performance of cellular communication in small-scale networks and large-scale networks, respectively.

In this chapter, we have presented physical layer (PHY)-security of cognitive radio network (CRN). Specifically, we have described fundamental PHY-security in CRN and pointed out some recent enhanced protocols available in the literature. Typical applications of artificial noise for the primary system, secondary system, and cooperative CRN have been investigated. We have also presented a powerful technique such as beamforming design on resource allocation problems for such schemes. For the primary system, based on the derived capacity formula, the impact of the secondary system on the secrecy capacity is analysed. In particular, we point out that when the eavesdropper is very far from the primary system, the use of artificial noise is not effective to protect the primary system from eavesdropping. For the secondary system, the proposed approach offers a better performance and is quite robust when compared to the existing approaches. In addition, a cooperative CRN is also presented to improve PHY-security of the primary system. Simulation results are shown to verify the theoretical developments.

Recent research has shown that millimetre wave (mmWave) communications can offer orders of magnitude increases in the cellular capacity. However, the physical layer secrecy performance of a mmWave cellular network has not been investigated so far. Leveraging the new path-loss and blockage models for mmWave channels, which are significantly different from conventional microwave channels, this chapter studies the network-wide physical layer security performance of the downlink transmission in a mmWave cellular network under a stochastic geometry framework. We first study the secure connectivity probability and the average number of perfect communication links per unit area in a mmWave network in the presence of non-colluding eavesdroppers. Then, the case of colluding eavesdroppers is studied. Numerical results demonstrate the network-wide secrecy performance, and provide interesting insights into how the secrecy performance is influenced by network parameters.

Directional modulation (DM), as a promising keyless physical-layer security technique, has rapidly developed within the last decade. This technique is able to directly secure wireless communications in the physical layer by virtue of the property of its direction-dependent signal-modulation-formatted transmission. This chapter reviews the development in DM technology over recent years and provides some recommendations for future studies.

The fifth generation (5G) of cellular systems addresses challenging objectives in terms of connection speed, latency, energy consumption, traffic type, number of served users and their type (not just humans but mostly devices or even herds of animals). In this plethora of requests, a proper design of the waveforms is seen by many in the field as a key factor for the success of the new standard. This chapter will look at the possible waveform candidates from a physical layer security (PLS) point of view, providing an overview of on-going research activity on how they can be exploited to either secretly transmit a message on a wireless channel or extract a secret key between a couple of users.

Non-orthogonal multiple access (NOMA) has been widely considered as a promising technology to enable high-efficient wireless transmissions in future 5G communication systems. In this chapter, we investigate single-input single-output (SISO) NOMA systems from the perspective of physical layer security. There into, two different SISO NOMA systems are studied in sequence so as to explore security issues in NOMA. First, we attempt the physical layer security technique in a SISO NOMA system which consists of a transmitter, multiple legitimate users and an eavesdropper who aims to wiretap the messages intended for all legitimate users. The objective is to maximise sum of secrecy rates subject to an individual quality of service constraint for each legitimate user, respectively. The investigations in this system will provide a preliminary analysis of the secure performance of SISO NOMA systems. Second, on the basis of the SISO NOMA system previously studied, a multi-antenna jammer is additionally equipped to enhance secure transmissions of the system. The joint optimisation of power allocation and beamforming design is considered. The efforts in this second system aim to propose an effective solution for realising secure transmissions for each legitimate user.

In this chapter, we consider a general scenario with a hybrid time and spatial AN-injection scheme. In particular, we consider the MIMOME-OFDM wiretap channel, where each node is equipped with multiple antennas and the legitimate transmitter (Alice) has perfect knowledge of the CSI for the wireless links to her legitimate receiver (Bob) only, while the eavesdropper (Eve) has perfect CSI knowledge of all the links in the network. This global CSI assumption at Eve represents the best case scenario for Eve (the worst case scenario for Alice/Bob) since she knows all the channels between Alice and Bob as well as the used data and AN precoders at Alice. We exploit the spatial and temporal degrees of freedom provided by the available antennas and by the cyclic prefix (CP) structure of OFDM blocks, respectively, to confuse Eve and, hence, increase the secrecy rate of the legitimate system.

Part V: Applications of physical layer security

Physical layer security may play an important role for establishing security in future communication systems in a plug-and-play manner. This particularly holds for the IoT, where special constraints, such as the sheer number of things to be securely inter-connected or the resource-constrained nature of many devices, make it hard to directly make use of existing approaches.

This chapter reviewed the key generation from wireless channels and presented a case study by implementing an RSS-based key generation system. We introduced the key generation principles, evaluation metrics, procedure, and channel parameters. We then implemented a key generation system by using WARP hardware, which is a customized FPGA-based platform. We carried out several experiments in the indoor environment and tested the key generation principles, i.e. temporal variation, channel reciprocity, and spatial de-correlation. We concluded that key generation is workable in dynamic environment but cannot operate properly in static channels.

The main objective of this chapter is to study explicit key extraction techniques and algorithms for the security of radio communication. After some recalls on the main processing steps (Figure 19.1(a)) and on theoretical results relevant to the radio wiretap model (Figure 19.1(b)), we detail recent experimental results on randomness properties of real field radio channels. Furthermore, we detail a practical implantation of secret key generation (SKG) schemes, based on the Channel Quantization Alternate (CQA) algorithm helped with channel decorrelation techniques, into modern public networks such as WiFi and radio-cells of fourth generation (LTE, long-term evolution). Finally, through realistic simulations and real field experiments of radio links, we analyze the security performance of the implemented SKG schemes, and highlight their significant practical results and perspectivesfor future implantations into existing and next-generation radio standards.

The objective of this chapter is to study practical coding techniques to provide security to wireless systems. First, the chapter will briefly introduce theoretical results relevant to low density parity check (LDPC) codes, polar codes and lattice coding for the wiretap channel. Then, it will propose practical secrecy-coding schemes able to provide a reliable and confidential wireless communication link betweenAlice and Bob. Finally, these practical wiretap codes are implemented in WiFi and long-term evolution (LTE) testbeds, and their confidentiality performance is evaluated using the bit-error rate (BER) as it is a simple and practical the metric for secrecy. The reader is referred to [1] for a throughout survey on recent advances related to the design of wiretap codes for information-theoretic metrics such as strong secrecy and semantic secrecy.